1. Field of the Invention
The present invention relates to an image forming apparatus. In particular, the invention relates to an electrophotographic image forming apparatus wherein two-color images are transferred onto an image bearing medium from an image carrier after successive development of the two electrostatic latent images on the image carrier with various color developers.
2. Discussion of Background
In this type of image forming apparatus, a first electrostatic latent image is formed by a laser beam on a photosensitive body as an image carrier and this first latent image is converted into a visible color image with a first color developer. Then, a second electrostatic latent image is formed by a second laser beam and this second latent image is converted into a visible color image with a second color developer. The visible images of the two colors are transferred at the same time onto a sheet.
A "Two-color printing laser beam printer" was disclosed by Hoshi et al. in the Gazo Denshi Gakkai-shi (Image Electronics Institute Journal) Vol. 13, No. 14 (1984). In the laser beam printer in the above disclosure, two-color images are obtained by reversal development using a known two-component developing process. However, this involves a problem, since a second latent image is formed without making the potential distribution of the first developed latent image uniform, and image portions of the first latent image become further developed by the second color developer at the second developing stage.
As against this, it has been made clear in the Journal of Imaging Technology, Volume 12, No.2, 1986: "Two-Color Recording Process for Electrophotographic Printer" (J. Nakajima et al.) that the above problem can be resolved by including an additional stage. In the additional stage, the potential distribution of the first latent image is made uniform by using a known scorotron charger between the first developing stage and the second developing stage. However, even when this is done, there is still a problem in that a portion of the first toner image that has been formed beforehand on the photosensitive body is wiped off in the second devloping stage, and toner of the first color contaminates the second developing unit, since the second development is effected by a contact developing method using magnetic toner.
In the Journal of Imaging Technology, Volume 12, No. 1, 1986: "High-Speed Color Laser Printing Process" (M. Kohyama et al.), the above problem is resolved by using non-contact development for the second development. This process makes it possible to completely avoid admixture of toner of different colors, whether on the photosensitive body or in developing units. However, the following problems remain.
The drawbacks of the prior art will be explained with reference to FIGS. 1 to 3. To obtain a first visible image on a photosensitive body 2 with a reversal development method, positively charged toner 3 of a first color adheres to image portions A on photosensitive body 2 on which a first positive electrostatic latent image has been formed. However, the tone inevitably includes particles having opposite polarity. Thus, this negatively charged toner 4 adheres to non-image portions B of the first latent image, as shown in FIG. 1. Then, photosensitive body 2 is charged by a charger 6 before forming of a second electrostatic latent image on photosensitive body 2. When this is done, negatively charged toner 4 also becomes charged, as shown in FIG. 2. After charging, the second electrostatic latent image is formed on the surface of photosensitive body 2. Then, toner of a second color (not shown) is adhered to the image portion on photosensitive body 2 with a reversal development method to obtain a second visible image on photosensitive body 2.
After development, the first and second visible images are transferred onto a sheet 5 by charging the sheet 5 with a negative polarity opposite to the positive polarity of toner 3. Thus causes electrostatic adhesion of positively charged toner 3, as shown in FIG. 3.
Since negative toner 4 also becomes positively charged by charger 6, negative toner 4 is also transferred onto sheet 5 with positive toner 3. As a result, background fogging undesirably occurs on the sheet after transfer of the first and second visible images onto the sheet.
In other words, there is a problem in that it is not possible to produce clear images with a conventional multi-color electrostatic recording apparatus.